Currently, metal gate electrodes are being investigated to replace polysilicon gate electrodes in today's ever shrinking and changing semiconductor devices. Traditionally, a polysilicon gate electrode with an overlying silicide was used for the gate electrodes in CMOS devices. However, as device feature size continues to shrink, poly depletion becomes a serious issue when using polysilicon gate electrodes. The polysilicon gate electrodes may be replaced with metal gate electrodes in order to solve problems of polysilicon-depletion effects and boron penetration when forming semiconductor device features.
Recently, silicided metal gates have been utilized in semiconductor device components, such as gates. Typically, in forming silicided gates, polysilicon is deposited over a gate dielectric. Then, a metal is deposited over the polysilicon and reacted to completely consume the polysilicon resulting in a fully silicided metal gate, in lieu of a deposited metal gate. The silicided metal gate provides a metal gate with the least perturbation to the conventional process and avoids contamination issues. Furthermore, polysilicon doping has been shown to affect the work function of the silicided metal gates.
According to the conventional process, the gates and other semiconductor components such as moats must be silicided at the same time. However, for fully silicided gates, the gates and other components cannot be silicided simultaneously because the fully silicided gate will require a different metal or different deposition thickness than other components.